Various honeycomb structures made of foldable material are disclosed in the prior art. For example, U.S. Pat. Nos. 4,450,027, 4,603,072 and 4,631,108 to Colson disclose a honeycomb structure constructed by folding a continuous length of foldable material at diametrically opposite sides partially over itself into a tubular form as an individual cell. In addition, U.S. Pat. No. 4,677,012 to Anderson discloses a honeycomb structure formed of a continuous length of foldable material having its longitudinal edges folded over one side of the material and secured to a separate strip material. In this patent the cells are also formed one by one and an additional strip material is required. Another U.S. Pat. No. 4,631,217 to Anderson discloses a honeycomb structure formed of a continuous length of foldable material which is folded into a Z-configuration. In this patent each cell is constituted at a front face by a piece of material and at a rear face by another piece of material, in which each piece of material is extended from one cell to an adjacent cell. Each, individual cell is formed by each one half of two pieces of material.
In order to overcome the disadvantages of lower productivity and inferior quality control, U.S. patent application Ser. No. 08/346,045 filed on Nov. 29, 1994 and assigned to the same assignee of this application provides a dual cell honeycomb formed of a continuous length of foldable material folded into a vertical 8-figured configuration, a plurality of which may be stacked and adhered to each other superposedly to form a window covering having a single column of cells in the structure. For enhancing the effective optical and thermal insulations and durable dimensional stability, a structure consisting of plural columns of cells is usually required.
U.S. Pat. No. 4,307,768 suggests an energy conserving insulative window shade essentially consisting of a multiplicity of parallel hollow channels formed by a plurality of initially parallel layers and adhered along the edges of the shade. In this shade the adhesion lines are visually exposed and always formed in a slovenly appearance, which is obviously not preferred for use in the window coverings.